1. Field of the Invention
This invention relates to a picture encoding method and apparatus, a picture transmitting method and a picture recording medium that may be decoded by a picture decoding apparatus. More particularly, it relates to a picture encoding method and apparatus, a picture transmitting method and a picture recording medium employed in a system for storing image signals of a moving picture in the form of codes for storage on a picture recording medium, such as an optical disc, a magnetic disc or a magnetic tape.
2. Description of the Related Art
In digitally recording picture data on a magneto-optical disc or a magnetic tape, or transmitting the picture data via a pre-set transmission medium, the customary practice is to encode data for compressing and diminishing the data volume.
The case of using the encoding standard by the Moving Picture Image Coding Experts Group (MPEG) as the encoding standard is explained. The MPEG, discussed in ISO-IEC/JTC1/SC2/WG11 and proposed as a standard, is of a hybrid system consisting in the combination of motion-compensated predictive encoding and discrete cosine transform (DCT), as proposed in, for example, the U.S. Pat. No. 5,155,593 by the present Assignee.
In the encoding by the MPEG system, each video sequence is divided into plural GOPs on, for example, the 15-frame basis. Each frame is classified by the prediction system into three sorts of picturers, namely an intra-frame coded picture, encoded within a frame itself, a predictive-coded picture or a P-picture, in which motion prediction is made from past and future P- or I-pictures, and a bidirectionally predictive-coded picture or B-picture, in which motion prediction is made from a past or future P- or I-picture.
In the data compression method according to the MPEG system, a picture of each frame is predicted in accordance with the prediction mode for the I-picture, P-picture or the B-picture, and the prediction error is encoded and transmitted. Basically, only the prediction error is transmitted, thus enabling the data volume to be compacted as compared to the case in which picture data of each frame is transmitted directly.
FIGS. 1A and 1B show a typical structure of a GOP (group-of-pictures) as a picture group structure including at least one I-picture. The frame number in the GOP denotes the display sequence.
In FIGS. 1A and 1B, image signals of 15 frames of from frame F0 to F14 make up a GOP (group-of-pictures) as a processing unit. In the drawings, frames denoted as "I", "P" and "B" denote frames encoded as the above I-, P- and B-pictures, respectively. For a frame of the I-picture, the picture information is encoded by itself and transmitted (intra-frame coding). For a frame of the P-picture, basically a frame of an I- or P-picture temporary previous to a picture under consideration is used as a prediction picture, and prediction residual signals are encoded and transmitted (forward or backward predictive coding). For a picture of a B-picture, basically a reference temporally past frame and a temporally future reference picture are used as prediction pictures and prediction residual signals are encoded and transmitted (bidirectional predictive coding).
If, in a series of transmitted pictures, a scene change, in which a scene or background is changed, occurs, and a picture after the scene change is predicted from a picture before the scene change, the prediction error becomes very large, such that the predicted picture is degraded in picture quality. If the frame thus degraded in picture quality is used as a prediction picture in compressing picture data of an other frame, picture data of such other frame is also degraded in picture quality.
Thus, in the conventional practice, an I-picture is inserted directly after scene change, as shown in FIGS. 2A and 2B, and such new I-picture is used as a reference frame for the subsequent processing for preventing propagation of picture quality degradation.
That is, if scene change SC occurs in a frame F7, a picture a of a frame F8 directly succeeding the scene change is changed to an I-picture, as shown in FIG. 2B.
However, in this case, the number of I-pictures is increased with the increase number of scene changes, resulting in an increased number of I-pictures in a given sequence. Since image signals for one frame are directly transmitted for the I-picture, a large information volume is required for transmission, such that the increased number of the I-pictures leads to lower compression efficiency and degraded picture quality.